The field of the invention is magnetic resonance imaging (MRI) and, in particular, a system for analyzing gradient usage when oblique MRI images are obtained.
When human tissue is subjected to a polarizing magnetic field B.sub.0, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field but precess about it at a Larmor frequency dependent principally on the magnetic field strength. The precession produces a resonance signal that may be detected and used to generate an image by imparting a spatially dependent phase and frequency to the spins through the use of superimposed and orthogonal gradient fields (G.sub.x, G.sub.y and G.sub.z).
The gradient fields are employed, individually or together, to first select a slice volume in which the spins are coherent by generating a functional "select" gradient F.sub.s in conjunction with an RF excitation. Second, within this slice, by means of a functional "frequency" gradient (F.sub.f), the frequency of the spins is changed according to their location along the axis of the F.sub.f gradient. Finally, along an axis orthogonal to that of the F.sub.s and F.sub.f gradients, a progressively increasing functional "phase" gradient (F.sub.p) is applied to vary the phase of the spins according to their location along that axis. The time sequence of data for a set of different phase encodings produces an array of data that when operated on by a two-dimensional Fourier transform yields an image of a slice through the patient. A basic overview of MRI image reconstruction is contained in the book "Magnetic Resonance Imaging, Principles and Applications" by D. N. Kean and M. A. Smith hereby incorporated by reference.
The functional gradients, F.sub.s, F.sub.f, and F.sub.p, need not be aligned with the physical gradients, G.sub.x, G.sub.y and G.sub.z, but may be oriented arbitrarily by the simultaneous energization of the various physical gradients G.sub.x, G.sub.y and G.sub.z whose vector sum can produce functional gradients along any arbitrary axis. When F.sub.s, F.sub.f, or F.sub.p are not aligned with any of the physical gradients G.sub.x, G.sub.y or G.sub.z, the image produced is termed "oblique". Oblique imaging is preferred for the study of certain parts of the body for diagnostic reasons.
As a general matter, it is desirable to be able to produce gradient fields with rapid rise and fall times ("high slew rate") and with high amplitude. Higher gradient slew rate generally allows shorter acquisitions of the necessary MRI signals. Higher amplitude gradient fields can also reduce the time required to obtain an MRI signal, for example, in conjunction with an increase of the bandwidth of the NMR receiver. Further, stronger gradient field also increase the spatial resolution of the imaging process permitting smaller voxels of a patient to be discerned. Generally, both the slew rate and amplitude of the gradients is determined by the maximum voltage and current of the gradient amplifiers powering the gradient coils.
In oblique imaging, the limitations of the gradient slew rate and amplitude are exacerbated by a need to limit the maximum value of functional gradient so that the physical gradients do not exceed their limits. The reason for this limitation is that a single physical gradient may be simultaneously called upon to provide components of two functional gradients and thus potentially be overtaxed. Accordingly, the peak functional gradients for oblique orientations are normally limited to be a fraction of the peak gradients that may be obtained for non-oblique views.